1. Field of the Invention
The present invention relates to the recycling of polyamide material, and in particular, to the separation of the useful reaction products of the ammonolysis of polyamide material.
2. Description of the Prior Art
It has long been considered ecologically desirable to reclaim polyamide material from used products, such as carpets, and to incorporate such reclaimed polyamide material into articles requiring high quality polymer. U.S. Pat. No. 5,302,756 and U.S. Pat. No. 5,395,974, both to McKinney and both assigned to the assignee of the present invention, relate to the ammonolysis of polyamide material into its constituent monomers.
FIG. 1 is a schematic diagram in which an ammonolysis reactor R in accordance with the above-mentioned patents is connected by a line L to a distillation system S. The distillation system S is configured in accordance with the teachings of the prior art for separating into fractions the output liquid stream from the ammonolysis reactor R.
In the method described in the referenced McKinney patents polyamide material in the form of nylon 6, nylon 6,6, or a mixture thereof, is charged into the inlet port I of the reactor R. Within the reactor R the polyamide material is reacted with at least one equivalent of ammonia per amide group at temperatures in the range between two hundred fifty and four hundred seventy degrees Celsius (250 to 470xc2x0 C.) and a pressure of at least one hundred (100) psig. The reaction is preferably carried out in the presence of certain Lewis Acid catalyst precursors.
The output from the ammonolysis reactor R, typically in liquid form, is conveyed by the line L to the distillation system S. The stream of ammonolysis products is a mixture of various useful reaction products as well as other, volatile, materials. These useful reaction products include:
(I) monomer product(s) having amine functionality, hereinafter referred to in this description as xe2x80x9camine-functional monomer product(s)xe2x80x9d. These amine-functional monomer product(s) have first, relatively high, predetermined vapor pressure(s) associated therewith;
(II) reactive polyamide material(s), hereinafter referred to in this description as xe2x80x9chigh boiling polyamide intermediate material(s)xe2x80x9d, or xe2x80x9chigh boilersxe2x80x9d having second, relatively lower, predetermined vapor pressure(s) associated therewith; and
(III) reactive monomer product(s) having vapor pressure(s) intermediate the first and second predetermined vapor pressures. These materials are hereinafter referred to in this description as xe2x80x9creactive monomer product(s)xe2x80x9d.
The other, volatile, materials include compounds, such as ammonia, water and carbon dioxide, having vapor pressures greater that the first-predetermined vapor pressure(s). These materials are hereinafter referred to in this description as xe2x80x9clow boiling volatile product(s)xe2x80x9d or xe2x80x9clow boiler(s)xe2x80x9d (xe2x80x9cLBxe2x80x9d).
These reaction products and volatile materials contained in the output stream from the reactor R are listed in FIG. 1 in descending order of vapor pressures and in ascending order of boiling points.
The identity and relative quantity of the products output in the output line L is dependent upon the polyamide materials introduced into the ammonolysis reactor R.
If nylon 6,6 is charged into the ammonolysis reactor R the amine-functional monomer product in the output stream is hexamethylene diamine (xe2x80x9cHMDxe2x80x9d) and the reactive monomer product is adiponitrile (xe2x80x9cANDxe2x80x9d). If nylon 6 is charged into the reactor R then the output stream contains a second, different, amine-functional monomer product, 6-aminocapronitrile (xe2x80x9cACNxe2x80x9d), and a second, different, reactive monomer product, caprolactam (xe2x80x9cCLxe2x80x9d). Of course, if a mixture of both nylon 6,6 and nylon 6 is charged into the reactor R, then the output stream includes the amine-functional monomer product and the reactive monomer product generated by each type of nylon.
Amine-functional monomer products have a predetermined vapor pressure generally lying in the range from about twenty millimeters Mercury at one hundred twenty degrees Celsius (20 mm Hg @ 120xc2x0 C.) to about eighty millimeters Mercury at the same temperature (80 mm Hg @ 120xc2x0 C.). The vapor pressure of hexamethylene diamine is on the order of about eighty millimeters Mercury at one hundred twenty degrees Celsius (80 mm Hg @ 120xc2x0 C.), while the vapor pressure of 6-aminocapronitrile is on the order of about twenty one millimeters Mercury at the same temperature (21 mm Hg @ 120xc2x0 C.).
The high boiling polyamide intermediate material(s), which include polyamide intermediates, primary amides and nylon dimers and oligomers, have vapor pressures lower than the vapor pressure of the amine-functional monomer products, generally lower than two millimeters Mercury at one hundred twenty degrees Celsius (2 mm Hg@ 120C.).
Generally speaking, the vapor pressures of reactive monomer products are intermediate the vapor pressures of the high boiling polyamide intermediate material(s) and the amine-functional monomer products. That is to say, the reactive monomer product(s) have a vapor pressure lower than the vapor pressure(s) of the amine-functional monomer product(s) and higher than the vapor pressures of the high boiling polyamide intermediate material(s). For example, the vapor pressure of adiponitrile is on the order of 2.4 millimeters Mercury at one hundred twenty degrees Celsius (2.4 mm Hg @ 120xc2x0 C.). The vapor pressure of caprolactam is on the order of six millimeters Mercury at the same temperature (6 mm Hg @ 120xc2x0 C.).
These various output products from the ammonolysis reactor R must be separated from each other in order for them to be purified to the degree-necessary to permit their re-use. Distillation is a traditional mode of separation of commingled materials based upon their relative vapor pressures. Illustrated in FIG. 1 is a distillation system S based upon traditional prior art distillation teachings, such as those set forth by Malone et al., xe2x80x9cSimple, Analytical Criteria for the Sequencing of Distillation Columnsxe2x80x9d, AIChE Journal, April 1985, 683.
As a general rule, the basic heuristic observed for sequencing distillation columns is to remove the lightest components first. The distillation system S shown in FIG. 1 implements this heuristic. The system S is a four-pass system configured from four cascaded distillation columns A through D, respectively. These distillation columns A through D separate the ammonolysis products in a sequential order corresponding to their vapor pressures.
In operation, the commingled ammonolysis products in the output line L are fed into the first column A where the low boiling impurities are removed as distillate. The tails, or bottoms, stream from the first column A is fed into the second column B to remove as distillate the amine-functional monomer product(s), hexamethylene diamine and/or 6-aminocapronitrile, as the case may be. The reactive monomer product(s), such as caprolactam and/or adiponitrile, as the case may be, are removed as the distillates from the third and fourth columns C and D, respectively. The high-boiling polyamide intermediate materials (xe2x80x9cHBxe2x80x9d) in the tails stream from the fourth column D contain amides, dimers, oligomers and tars. As used herein both the term xe2x80x9ctailsxe2x80x9d and the term xe2x80x9cbottomsxe2x80x9d refer to the stream taken from the bottom of a given distillation column.
The quantitative results of the operation of a four-column distillation system S as illustrated and described in connection with FIG. 1 is here set forth as a Comparative Example. It should be noted that the distillation columns A through D as shown in FIG. 1 were run as individual experiments and not as a continuous process.
The same column was.used for all four separations. The column was two (2) inches in diameter and packed with five (5) feet of 0.16 inch metal protruded packing. The feed point was located two (2) feet from the bottom of the packed column in the first two columns, and just above the reboiler in the last two columns. The reboiler was a twenty-two (22) liter pot heated by an electric heating mantle.
The operating parameters for the column when emulating the operation of each of columns A through D of FIG. 1 were as follows:
Although the distillation columns were run as individual experiments and not run as continuous process an overall recovery of distillation system S as configured in FIG. 1 can be calculated by multiplying the recoveries in each step for each individual monomer.
The recovery of each monomer was calculated as follows:
For a monomer in the distillate:
% Recovery of Monomer=(flow rate of Monomer in distillate+flow rate of Monomer in cold traps)xc3x97100
(flow rate of Monomer in feed)
For a monomer in the tails stream:
% Recovery of Monomer=(flow rate of Monomer in tails stream)xc3x97100
(flow rate of Monomer in feed)
The % Loss or Gain of each monomer was calculated as follows:
% Loss or gain of Monomer=(flow rate of Monomer in distillate, tails, cold trapsxe2x88x92flow rate of Monomer in feed)xc3x97100
(flow rate of Monomer in feed)
The overall recovery of the four monomers, viz., hexamethylene diamine, 6-aminocapronitrile, adiponitrile and caprolactam, in this example was sixty-seven percent (67%).
The results are summarized in the following Table 1.
The low recoveries from the four-column prior art distillation system S of FIG. 1 is believed to be due to the reactive nature of the ammonolysis products. Reactions between the amine-functional monomer product(s) and the primary amides in the high boiling polyamide intermediate materials limit the recovery rates for hexamethylene diamine and 6-aminocapronitrile. In addition, the amine functionality in the high boiling polyamide intermediate materials catalyzes the thermal degradation of adiponitrile, which limits the recovery of this reactive monomer product.
In view of the foregoing, it is believed advantageous to have a more efficient separation process in which reactions among the various ammonolysis products are reduced, with a concomitant increase in the recovery of the desirable monomer products.
The present invention is directed toward a process for separating into fractions an output stream from an ammonolysis reactor. The output stream includes the useful reaction products:
(I) amine-functional monomer product(s);
(II) high boiling polyamide intermediate material(s);
(III) reactive monomer product(s).
The output stream also may include:
(IV) low boiling volatile product(s).
The present invention is based upon the recognition that it is the prolonged presence of the high boiling polyamide intermediate material(s) in the distillation system in contact with the amine-functional monomer product(s) or with the reactive monomer products that limits the recovery rates of these useful monomer products.
Therefore, in accordance with a first embodiment of the present invention, as the first treatment of the output stream that separates at least one of these useful reaction products from the other reaction products, the high boiling polyamide intermediate material(s) are separated from both the amine-functional monomer product(s) and the reactive monomer product(s).
In accordance with a first implementation of the first embodiment of the invention the amine-functional monomer product(s) and the reactive monomer product(s) are taken as a distillate stream from a first distillation column. The high boiling polyamide intermediate material(s) separate in the tails from the first distillation column.
In an alternate implementation of the first embodiment of the invention, as the first treatment of the output stream from the ammonolysis reactor that separates at least one of the useful reaction products from the others, the amine-functional monomer product(s) are separated as the distillate stream from a first distillation column, while the reactive monomer product(s) are taken as a side draw from the column. The high boiling polyamide intermediate material(s) again separate in the tails from the distillation column.
Depending upon the polyamide material charged into the ammonolysis reactor the amine functional monomer product(s) may be hexamethylene diamine and/or 6-aminocapronitrile, while the reactive monomer product(s) may be adiponitrile and/or caprolactam. In more particularized versions of either implementation of the first embodiment of the invention, additional distillation columns are provided to separate the plural amine functional monomer product(s) from each other and to separate the plural reactive monomer product(s) from each other.
In accordance with a second embodiment of the present invention, as the first treatment of the output stream that separates at least one of these useful reaction products from the other reaction products, the high boiling polyamide intermediate material(s) together with the reactive monomer product(s) are separated from the amine-functional monomer product(s). As an immediately following second step, the reactive monomer product(s) are separated from the high boiling polyamide intermediate material(s).